Regular stereoscopic guidance is done using two predefined angular positions of an X-ray tube for X-ray images, wherein one angular position is left and the other one is right of the centre position. The two coordinates of a specific lesion in the two images can be used to identify all three coordinates of the lesion in the object of examination. To identify the z-position of a lesion, a cross hair cursor has to be set in the two stereo images. The z-position can be calculated from these values. However, the accuracy of the x-position (horizontal) may propagate into an uncertain z-position. A tissue sample can be taken at this position using a biopsy needle.
In tomosynthesis imaging several x-ray images are acquired in different angles from a subject volume, such as a part of the human body. From this set of projection images a three-dimensional data set can be derived from which two-dimensional images can be reconstructed.
Such a tomosynthesis imaging for diagnosing breast cancer is described in US 2007/0225600 A1. This document relates to a system and a method for the investigation of a body volume, particularly for the diagnosis of breast cancer. A sequence of X-ray projections from different directions is produced by a rotatable X-ray source and a stationary digital X-ray detector. From these projections, a set of sectional images is calculated by tomosynthesis. A physician may indicate a suspicious structure on a reference image that is derived from one of the projections or sectional images and displayed on a monitor. The computer may then locate the structure on all sectional images and calculate the similarity of a corresponding image feature. The sectional image at which the similarity is strongest then indicates the depth at which the structure is positioned in the body volume. Based on this information, a biopsy device with a needle can be advanced into the body volume until it reaches the suspicious structure.
However, in these systems some of the projection images may show a shadow of the biopsy device if the biopsy device is located within the X-ray beam. FIG. 1 shows a stereoscopic or tomosynthetic imaging system. On the upper left side, the system is shown in a right angular position, and below, there is schematically shown a resulting projection image which illustrates some circular lesions, a shadow of the biopsy needle holder 1, a shadow of the biopsy needle 2 and part of an opening 3 of a biopsy paddle. On the upper right side, the system is shown in its left angular position, and below, there is also schematically shown a resulting projection image which also illustrates some circular lesions and the shadow of the biopsy unit, i.e. the biopsy needle holder shadow 1, the biopsy needle shadow 2 and the shadow of the opening 3.